It has been demonstrated that the ability of lymphoid populations to generate cell mediated cytotoxicity against alloantigens in vitro can be completely abrogated by the depletion of adherent accessory cells. Activity is totally restored in these functionally depleted populations by the addition of small numbers of radioresistant theta negative peritoneal cells (macrophages). In addition it has been demonstrated that this accessory cell function can also be reconstituted by tissue culture maintained syngeneic tumor cells. These findings are interpreted as demonstrating that cells other than adherent macrophage like cells are capable of providing accessory cell function for cell mediated immune responses in vitro.